Mixed matrix membranes (MMMs) are gas separation membranes that attempt to integrate the outstanding separation properties of molecular sieves (zeolites, carbon molecular sieves, etc.) with the cost and processing advantages of polymers. Ideally, MMMs would provide a solution to the permeability/selectivity tradeoff observed for polymer membranes and expand the market for membrane separations versus more traditional absorption- and adsorption-based operations. Such improvement in membrane performance could prove especially beneficial in high-volume gas separation areas such as natural gas processing.
Natural gas processing currently represents the largest market for industrial gas separations; this is likely to remain true as global natural gas consumption increases towards an estimated 150 trillion standard cubic feet annually by 2030. Virtually all natural gas requires some degree of processing prior to transport via pipeline or tanker. For example, a common contaminant found in gas wells—carbon dioxide—often must be substantially removed to prevent pipeline and equipment corrosion and line blockage due to solid carbon dioxide accumulation. The removal of carbon dioxide from natural gas streams, however, may be especially troublesome for polymer membranes due to, for example, the strong plasticization response of most commercially applicable polymers. This phenomenon may result in deleterious membrane performance loss.
Crosslinking may stabilize some polymer membranes against plasticization to some degree and assist in maintaining membrane performance under aggressive feed conditions. However, crosslinked polymer membrane separation performance may be limited by the permeability/selectivity tradeoff. Also, MMMs utilizing crosslinkable polymer matrices are more beneficial if they exhibit adequate adhesion at the polymer/sieve interface. This assists in reducing non-ideal interfacial morphologies commonly referred to as sieve-in-a-cage and leaky interface defects. Such a reduction would increase polymer/sieve compatibility and foster MMMs goal of enhancing permeability and selectivity.
A number of methods aimed at improving polymer/sieve adhesion have been attempted. For example, thermal annealing did not achieve complete elimination of interfacial voids and also sometimes resulted in thermal degradation. Silane coupling agents may result in little to no improvement in transport properties for certain materials, as well, some gas leakage at the interface. And unfortunately modifying the surface of zeolite A by roughening the surfaces of modified sieves with nanoscale deposits of magnesium hydroxide is complex and may not be highly effective with, for example, high-silica molecular sieves.
Thus, what is needed is a way of overcoming the aforementioned limitations and improving the performance of crosslinkable MMMs in, for example, gas processing. It would further be beneficial if such a procedure and MMM did not require, for example, silane coupling agents or inorganic nanoscale surface deposits. The procedure and MMM would be further beneficial if it enhanced the MMM's permeability, selectivity, and/or if the crosslinked membranes were stable even under highly-plasticizing feed conditions as compared to prior art MMMs.
Advantageously, this invention overcomes many of the limitations of the prior art and has additional advantages. In one embodiment, the invention relates to a method for treating molecular sieve particles for use in a mixed matrix membrane. The mixed matrix membrane comprises the treated molecular sieve particles dispersed in a crosslinked polymer continuous phase. The method comprises: (a) selecting suitable molecular sieve particles; and then (b) polymerizing a polymer which is compatible with the polymer to be employed in the mixed matrix membrane in the presence of the molecular sieve particles to obtain treated molecular sieve particles.
In another embodiment, the invention pertains to a mixed matrix membrane for separating carbon dioxide and methane. The membrane comprises (1) a crosslinked polymer continuous phase; and (2) treated molecular sieve particles dispersed in said continuous phase. The membrane is characterized by having a carbon dioxide/methane ideal selectivity of at least about 5% higher at 65 psia and 35° C. than a comparable membrane with untreated molecular sieve particles.